Calculate totals for each unique 2nd level data set in a 3d array

I am trying to calculate the total of unique DRIVER values in a multidimensional array.

Input:

$distance_covered = [
    '1_JAN_2017' => ['DRIVER_1' => [2, 5, 3],    'DRIVER_2' => [3, 2, 6, 9]],
    '2_JAN_2017' => ['DRIVER_1' => [3, 9],       'DRIVER_3' => [1, 4, 8]],
    '3_JAN_2017' => ['DRIVER_4' => [9],          'DRIVER_1' => [2, 7, 5, 2]], 
    '4_JAN_2017' => ['DRIVER_1' => [5, 3, 3, 2], 'DRIVER_4' => [4, 9, 8, 5]], 
    '5_JAN_2017' => ['DRIVER_2' => [8, 5],       'DRIVER_5' => [3, 9, 7]],
    '6_JAN_2017' => ['DRIVER_5' => [2, 1, 7, 5], 'DRIVER_4' => [1, 9, 6]], 
    '7_JAN_2017' => ['DRIVER_4' => [5, 2, 9],    'DRIVER_3' => [4, 1, 6]],
]; 

Desired result:

[
    'DRIVER_1' => 51,
    'DRIVER_2' => 33,
    'DRIVER_3' => 24,
    'DRIVER_4' => 67,
    'DRIVER_5' => 34
]

This is the sum of distance travelled by each driver in all trips

I tried code like this:

$res = array();
foreach($distance_covered as $value) {
    foreach($value as $key => $number) {
        (!isset($res[$key])) ?
            $res[$key] = $number :
            $res[$key] += $number;
    }
}
print_r($res);

This one works for me

$res = array();
foreach($distance_covered as $value)//the array which you have given us 
{
    foreach($value as $key => $number) //loop over array of date
    {
        if(!isset($res[$key]))//check if the key exist in over defined array if no then run this
        {
            $res[$key] = array_sum($number);// Sum all distances of that driver
            continue;//set the key and continue the foreach... 
        }
        $res[$key] += array_sum($number);// Sum all distances of that driver    
    }
}

print_r($res);      
die;

And the Output is

Array
(
    [DRIVER_1] => 51
    [DRIVER_2] => 33
    [DRIVER_3] => 24
    [DRIVER_4] => 67
    [DRIVER_5] => 34
)